As the dimensions of transistors decrease, the thickness of the gate oxide must be reduced to maintain performance with the decreased gate length. However, in order to reduce gate leakage, high dielectric constant (high-k) gate insulator layers are used which allow greater physical thicknesses while maintaining the same effective thickness as would be provided by a typical gate oxide used in larger technology nodes.
Additionally, as technology nodes shrink, in some IC designs, there has been a desire to replace the typically polysilicon gate electrode with a metal gate electrode to improve device performance with the decreased feature sizes. One process of forming the MG electrode is termed “gate last” process in which the final metal gate electrode is fabricated “last” which allows for reduced number of subsequent processes, including high temperature processing, that must be performed after formation of the gate.
FIG. 1 illustrates a partial cross-sectional view of a conventional high-k/metal gate structure for a Field Effect Transistor (FET) 100. The FET 100 can be formed over an active region of the substrate 102 adjacent to isolation structures 104. The FET 100 includes lightly doped regions 112 and source/drain regions 114 formed in a portion of the substrate 102, a gate structure 101 comprising a gate dielectric layer 106 and a metal gate electrode sequentially formed over the substrate 102, and spacers 110 respectively formed on both sidewalls of the gate structure 101. Additionally, a contact etch stop layer (CESL) 116 and an interlayer dielectric (ILD) layer 118 may also be formed over the substrate 102. The metal gate electrode comprises a metal capping layer 120, a first metal layer 122, a metal barrier layer 128, and a second metal layer 130 sequentially formed over the gate dielectric layer 106. The first metal layer 122 comprising Al and Ti may act as a work-function metal layer of an n-type FET. It has been observed that the Al may diffuse into the metal barrier layer 128, and interact with the metal barrier layer 128 to create inter-metallic compounds 124. The inter-metallic compounds 124 are problematic. For example, any inter-metallic compound 124 present in the metal gate electrode can become a gate leakage path of the gate structure 101 thereby increasing the likelihood of device instability and/or device failure.
Accordingly, what is needed is a metal gate electrode of a gate structure having no leakage path.